1. Field of the Invention
The present invention mainly relates to a power amplifier for mobile communication such as a cellular phone.
2. Background Art
GaAs-HBT power amplifiers are widely used as power amplifiers for CDMA-based cellular phones or the like (e.g., see Japanese Patent Laid-Open No. 2004-343244). Conventional power amplifiers require a reference voltage to be inputted from outside. Since this reference voltage determines an idle current of the power amplifier, the reference voltage needs to be resistant to a fluctuation in a supply voltage and be kept constant with high accuracy (e.g., on the order of 2.85 V±0.1 V).
In recent years, reference voltages are required to be generated inside power amplifiers. In this case, a reference voltage is generated inside the amplifier and the power amplifier is operated according to an enable signal (digital signal to turn ON/OFF the power amplifier) given from outside (e.g., see Japanese Patent Laid-Open No. 2010-124408).
An idle current flowing through a low output transistor during a low output operation is on the order of several mA (e.g., 2 mA to 7 mA), which is smaller than that during a medium output (e.g., on the order of 15 to 30 mA) and during a large output (e.g., on the order of up to 120 mA). The size of the emitter electrode of a transistor that determines a maximum value of output power of a low output transistor is also much smaller, for example, equal to or less than one several tenths of that of transistors used in other operating modes (e.g., the area of the emitter electrode of the low output transistor is 20 to 40 um2). The size of a transistor used for a bias circuit can be much smaller (on the order of one-severalth to one-twentieth of that of the low output transistor). However, from the standpoint of constraints on a semiconductor process device and product reliability or the like, the size of the emitter electrode of a transistor is subject to a lower limit. Therefore, the size of a transistor used for a bias circuit may be slightly greater than a low output transistor (one-severalth of that of the low output transistor).